1. Field of the Invention
This invention relates, in general, to fixing devices. In particular, the invention relates to a fixing device, which is used in an image forming apparatus to fix a toner image on a paper sheet, including a fixing roller, a pressure roller engaged with the fixing roller and a cleaning pad which is in contact with the surface of the fixing roller to clean the surface of the roller.
2. Description of the Related Art
As is well known, a conventional image forming apparatus, e.g., electro-photo copying machines, laser beam printers, etc., typically includes an image forming unit for forming an image on an image carrier, a developing unit for developing the image, as a visible image, on the image carrier by a toner powder, and a transfer unit for transferring the developed image formed on the image carrier to a paper sheet. The image forming apparatus also includes a fixing unit which fuses the toner powder (developed image) on the paper sheet and fixes the fused toner on the paper sheet as a fixed image.
A conventional fixing unit is shown in FIG. 1. A fixing unit 11 includes a cylindrical fixing roller (heat roller) 13 in which a heater (not shown) is provided at a center portion along a length thereof. Each side of fixing roller 13 is rotatably supported by a suitable bearing means, respectively, and thus, fixing roller 13 is rotated in counter-clockwise by a motor (not shown). A pressure roller 15 is located below fixing roller 13 such that it is forcibly engaged with the surface of fixing roller 13 by a spring (not shown). Thus, pressure roller 15 is rotated in clockwise when fixing roller 13 is rotated as described above. As shown in FIG. 1, those rollers 13 and 15 are rotatably located in a housing 17. A lead-in opening 17a is opened in one side wall of housing 17 to introduce a paper sheet P into housing 17 and a lead-out opening 17b is also opened in the opposite side wall to discharge paper sheet P from housing 17. A cleaning unit 18 is slidably fitted in an opening 21 formed in the upper wall of housing 17. Cleaning unit 18 has a cleaning pad 19 therein. One end of pad 19 is in contact with the surface of fixing roller 13 at a prescribed pressure by a suitable pressing means, e.g., a leaf spring 23. One end of leaf spring 23 is fixed to an upper openable cover 25 of an image forming apparatus and the other end is in contact with cleaning unit 18 to urge cleaning pad 19 on the surface of fixing roller 13. Leaf spring 23 has relatively a long length to easily regulate the pressure on cleaning pad 19. Cleaning pad 19 is always urged on the surface of fixing roller 13 under the prescribed pressure.
A paper sheet P on which a developed toner image has been transferred is led into fixing unit 11 (housing 17) through lead-in opening 17a. When paper sheet P passes through the contact portion between fixing roller 13 and pressure roller 15, the toner on paper sheet P is fused by heat and pressure generated by rollers 13 and 15 and is fixed on paper sheet P as a fixed image. In this fixing operation, almost all fused toner is fixed on paper sheet P, but a part of fused toner on the surface of paper sheet P may be transferred to the circumferential surface of fixing roller 13. The transferred toner on fixing roller 13 is removed by cleaning pad 19.
Presently, there is a tendency to minimize the external size of an image forming apparatus. Thus, a compact size, in particular a reduced height of the fixing unit is desired to achieve a reduced external size of the image forming apparatus.
In the conventional fixing unit having the above-described construction, it may be difficult to reduce the height of the unit because of the leaf spring. If the length of leaf spring 23 is reduced, regulation of pressure generated by leaf spring 23 may be difficult. Pressure by leaf spring 23 is applied in excess to cleaning pad 19 resulting in the reduction of durability of cleaning pad 19. Otherwise, insufficient cleaning by cleaning pad 19 may occur. Since cleaning pad 19 of the conventional fixing unit is always urged on the surface of fixing roller 13 at a prescribed pressure, cleaning pad 13 might be deformed. Therefore, the service life of cleaning pad 23 is shortened.